1. Field of the Invention
This invention relates to an improved dental implant used in the field of dental therapy.
2. Description of the Related Art
Recently, extensive studies are being made on the dental implant technology as a prosthesis means for the treatment of dental fall off and damages. The purpose of this technology is to embed a dental implant having the functions equal to those of the original tooth in the position from which the tooth is fallen off or extracted because of the damage.
Conventionally, dental implants, made of titanium or titanium alloy, or single crystalline alumina have been widely employed. For the purpose of enhancing the affinity with bone it has been proposed in the art that, titanium or titanium alloy implants whose surfaces are to be in contact with the bone be covered with hydroxyapatite (HAP) or calcium phospate (TCP).
Most of these conventional dental implants, however, have no function corresponding to that of the periodontal membrane of natural tooth, especially the function of shock absorber. The dental implant which lacks the shock absorbing function, for example, an implant shown in FIG. 12, has been known. The implant of FIG. 12 comprises an implant body 3a made of titanium which is embedded in jaw bone, a gingiva-penetrating member 4 made of titanium which is engaged with an upper end portion of the implant body 3a, a post core 10 made of titanium which is screwed through the gingiva-penetrating member 4 into a post core fixing hole provided in the implant body 3a, an artificial tooth 8 fixed on the post core 10 using a securing pin 11, and a filler member 12 buried in a space formed above the securing pin 11. Such a dental implant allow direct transmission of the occlusal force to jaw bone 1, which gives a great stress to the jaw bone in case of excess occlusal force applied, resulting in damage to the jaw bone.
Unexamined Japanese Patent Publication 58-116353 also discloses a dental implant provided with a shock absorbing member between an inner crown and an outer crown.
In addition, Unexamined Japanese Patent Publication 62-38148 (corresponding to U.S. Pat. No. 4,713,006) discloses a dental implant in which a shock absorbing member made of polymeric material such as silicone rubber, polyoxymethylene (POM) and the like is provided between the implant body and the post core in order to absorb and reduce the impact force loaded on the tooth crown.
FIG. 13 shows one example of such a dental implant having a shock absorbing member. The dental implant of FIG. 13 has an implant body 3a embedded in jaw bone 1. A gingiva-penetrating member 4 of cylindrical shape is engaged with an upper end portion of the implant body 3a, and fixed by screwing a polymeric material member 5b into the implant body 3a through the gingiva-penetrating member 4. On a central portion of an upper end of the gingiva-penetrating member 4, there is provided a threaded hole into which a securing pin 11 is screwed, in order to fix an artificial tooth 8 by screwing the pin 11 into the threaded hole. The artificial tooth 8 has a space on the securing pin 11, in which space a filler member 12 is buried.
The dental implants having the shock absorbing system described above, however, have following problems.
The first problem is that the excessive movement of the crown due to a great deformation of shock absorbing member made of the polymeric material results in a poor occlusal force.
The second problem is that periodic replacement of the shock absorbing member is required in order to prevent degradation or destruction of the polymeric material.
For avoiding the occurrence of the first and the second problems described above, there has been proposed a dental implant wherein, as shown in FIG. 14, disk-shaped shock absorbing member 5c made of polymeric material is employed in order to prevent deterioration of the shock absorbing member. In FIG. 14, the shock absorbing disk 5c made of polymeric material such as polyoxymethylene is disposed on the gingiva-penetrating member 4 which is screwed into the implant body 3a to be fixed. Both the gingiva-penetrating member 4 and the implant body 3a are made of titanium. Then, artificial tooth 8 is fixed on the shock absorbing member 5c by screwing the securing pin 11 into the gingiva-penetrating member 4 through the disk 5c. As a result, the shock absorbing disk 5c is sandwiched between the gingiva-penetrating member 4 and the artificial tooth 8 to be fixed.
The dental implant of FIG. 14 can reduce the stress imposed on the shock absorbing disk because occlusal force imposed on the artificial tooth 8 is mainly assigned to the securing pin 11.
However, the dental implant shown in FIG. 14 has another problem in that the stress from occlusal force is focused on the neck of the securing pin 11, fixing the shock absorbing disk 5c so as to give residual strain to the securing pin 11, thereby resulting in deviation of the occlusal position.